An automotive wheel suspension system for a steerable wheel typically includes a knuckle rotatably supporting a wheel and configured to be actuated by a steering gear, various arms and links (simply referred to as “arms” hereinafter) connecting the knuckle to a vehicle body, a spring for cushioning the input from the road surface and a damper for damping the vertical oscillating movement of the wheel. The Macpherson strut type, double-wishbone type and few other types have been preferred as suspension systems for a steerable wheel. In particular, recent development efforts have been directed to various forms of double-joint type wheel suspension systems that provide an increased freedom in setting the king pin (see patent documents 1 and 2).
The double-joint wheel suspension system is considered as a modification of a Macpherson strut type or double-wishbone type wheel suspension; at least one of the upper and lower arms (typically consisting of an A-arm) of the latter is separated into two separate arms. The first arm is connected to the knuckle via a first ball joint, and the second arm is connected to the knuckle via a second ball joint. In a double-joint wheel suspension system, an instantaneous center of rotation of an imaginary king pin passes through an intersection between extension lines of the first and second arms. Therefore, the king pin offset (wheel center offset) can be minimized by tilting the king pin more than usual, and this contributes to the reduction in the sensitivity to external interferences and torque steer while improving the steering feel. A typical ball joint used in a double-joint wheel suspension system comprises a ball stud consisting of a ball portion and a shank portion, a socket that receives the ball portion and a nut that threads with an outer thread formed in the shank portion of the ball stud, and the shank portion of the ball stud is fixedly secured to an upwardly facing or downwardly facing surface of the knuckle.    patent document 1: Japanese patent laid open publication No. 63-232010    patent document 2: Japanese patent laid open publication No. 5-278420